Sulphur black is a popular black dye. Because it is very stable and does not fade, even under strong sunlight, it has become a major dyestuff, particularly used for the manufacture of black denim and similar cotton textiles. It is estimated that annual production of sulphur black dye is about 12,000 tons per year. Use of sulphur black dye is prominent in third world and developing nations because it is cheap and particularly suited for cotton textile production, e.g. black denim jeans.
Sulphur black dye is produced by heating 2,4-dinitrophenol with sodium polysulfide at elevated temperatures under pressure. The resultant "melt" may then be diluted with water and air blown to oxidize the reduced form to an insoluble dye. During air blowing, sodium thiosulfate is formed and remains in the mother liquor. A proposed structure for sulphur black dye involves a substituted dibenzthiazine. For the most part, sulphur black is water insoluble, but can be solubilized by various treatments.
While sodium thiosulfate finds extensive commercial use, it cannot be recovered from the mother liquor of the sulphur black formation process because significant amounts of sulphur black are retained in the mother liquor. Furthermore, when using sulphur black, the spent solutions used for dying require disposal. Since sulphur black is very dark and imparts a dark color to the aqueous medium in which it is present, dumping of sulphur black into bodies of water is undesirable. Unfortunately, sulphur black dye does not seem to be readily degraded or removed from these aqueous mediums by conventional sewage treatments.
In addition, the textile industry is often interested in removing a portion of the sulphur black dye present in recently dyed substrates, e.g. in the production of "stone-washed" clothing. Traditional methods for removing dye from substrates make use of bleaching reagents, such as hypochlorite. However, hypochlorite and similar bleaching reagents are considered by many governmental agencies to be harmful to the environment. Accordingly, many governmental agencies have either prohibited, or at least heavily regulated, their use.
Accordingly, there is substantial interest in finding ways to treat solutions which comprise unwanted sulphur black dye. For example, there is interest in finding ways to remove sulphur black dye from the sulphur black mother liquor so that the sodium thiosulfate can be recovered for further commercial application. There is also substantial interest in finding ways to treat the waste streams produced by textile dyeing facilities so as to remove and degrade unwanted sulphur black dye. Further, there is substantial interest in finding new ways to remove sulphur black from dyed substrates.